Zn-Al alloy excellent in elongation and method for producing the same

ABSTRACT

A Zn—Al alloy excellent in elongation, and a method for producing the same are provided. The Zn—Al alloy comprises Zn in a range of 68 to 88% by mass and the remainder including Al and unavoidable impurities, and has a structure with β phases finely dispersed in respective α phases or respective α′ phases, not more than 5 μm in average grain size, a macrosegregation value of Al in the structure being less than 3%, wherein lamellar structures at a central part of the structure is at not higher than 30% by volume, and a difference in average hardness between the central part and a surface layer zone of the structure is not more than 15%, so that the Zn—Al alloy is improved in respect of elongation and uniformity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a Zn—Al alloy excellent in elongation that can be used in various applications making the most of superplasticity at room temperature, including base isolation/vibration control devices such as a constructional damper, and so forth, an impact absorption member of an automobile, and so forth, a spring member having damping capacity, a precision-forming component, a sealing member, a gasket member, a foil member requiring superplasticity and deformation property at room temperature, a target member for thin-film formation, and so forth, and a method of producing the Zn—Al alloy.

2. Description of the Related Art

Attention has lately been focused on a Zn—Al alloy exhibiting superplasticity at room temperature and having no toxicity, capable of replacing a toxic Pb-bearing alloy as a metal for vibration control for use in devices small in size and light in weight.

In connection with such a Zn—Al alloy as described above, it has been reported that a Zn—22% Al alloy, in nano-crystal form, has superplasticity capable of responding to deformation at a strain rate of 1×10⁻⁴ S⁻¹ at 373 K (about 100° C.) (refer to,for example, Non-patent Document 1). However, such superplasticity as above has not been attained as yet at room temperature, so that the alloy described as above cannot, in reality, be used in a constructional device for base isolation.

Further, it has been disclosed that a Zn—22% Al—2% Cu alloy was water-cooled after soaking to be subsequently subjected to cold working, whereupon a structure with the β phase precipitated in the α phase was obtained, thereby having attained superplasticity at room temperature (refer to Non-patent Document 2). It is shown herein that elongation is at 135%, and the maximum elongation at 160% can be obtained. However, it is not shown in this document that the alloy when subjected to warm working has such elongation as described at room temperature. Furthermore, even in the case of cold working, more excellent superplasticity (base isolation performance and vibration control performance) is desired of the alloy as a substitute for a Pb-bearing damper, so that further improvement in elongation (elongation at, for example, 180% or higher) is required.

Still further, it has been reported that a Zn—22% Al alloy attaining superplasticity at room temperature was obtained with the use of an experimentally minute specimen (refer to Non-patent Document 3). More specifically, it has been disclosed that a Zn—22% Al alloy columnar in shape with an initial grain size of a metal structure thereof, being in a range of 1 to 15 μm, was subjected to intense twisting deformation (cold deformation) under a high pressure at 5 GPa, whereupon a grain size in the central part (the microscopically smallest part) of a final structure was found to be in a range of 0.1 to 0.5 μm.

According to such a method as described above, however, it is possible to obtain only a structure having a peripheral part considerably differing from the central part thereof in that a granular structure of the peripheral part, away from the central part, is coarse, exhibiting no superplasticity although the central part is found to be a microstructure having a possibility of exhibiting superplasticity. Further, since dimensions of a member to which the intense twisting deformation is applicable are limited to very small ones such as on the order of 15 mm in diameter, and 0.3 mm in thickness, so that in the case of a member to be subjected to a large load, such as a device for base isolation, it is difficult to turn the member in whole into such microstructures as described, so that it is not possible to obtain a member capable of exhibiting superplasticity in its entirety.

Meanwhile, the inventors have since conducted R and D on the Zn—Al alloy described in the foregoing from the viewpoint of improvement on the properties thereof, and as part of the R and D, have proposed a Zn—Al alloy at a level of a constructional member having a uniform and stable super-microstructure, and capable of exhibiting elongation equivalent to superplasticity at room temperature (refer to Patent Document 1).

As a result of development of such a technology as described above, the inventors have succeeded in production of the Zn—Al alloy of a commercial size, exhibiting superplasticity at room temperature. The alloy developed as above is excellent in deformability (hereinafter referred to also as “static deformability”) at a low strain rate on the order of 1×10⁻³ S⁻¹, exhibiting excellent superplasticity at room temperature. However, there have been encountered cases where it was not possible to stably obtain deformability (hereinafter referred to also as “dynamic deformability”) at a relatively high strain rate on the order of 1×10⁻¹ S⁻¹. Further, occurrence of this phenomenon was found more pronounced as an ingot increases in size.

In order to improve the dynamic deformability to cope with the above-described, there has been proposed to aim at reduction in coarse Al-based inclusions, macrosegregation of Al and microsegregation of Al in addition to control of the super-microstructure according to a technology proposed under Patent Document 1 (refer to Patent Document 2).

Still further, a proposition has been made that as to pores unavoidably formed in an alloy structure during a production process of the alloy, presence of the pores 0.5 mm or greater in diameter is to be prevented to thereby obtain excellent dynamic deformability even without imposing strict control on the super-microstructure such that an average grain size is not more than 0.05 μm (refer to Patent Document 3).

[Patent Document 1]

JP-A No. 222643/1999 (full test)

[Patent Document 2]

JP-A No. 162103/2004 (full test)

[Patent Document 3]

JP-A No. 194541/2005 (full test)

[Non-patent Document 1]

“The observation of tensile superplasticity in nanocrystalline materials” by R. S. Mishra, et al., Nanostruct Mater. Vol. 9, No. 1/8, p. 473-476 (1997)

[Non-patent Document 2]

“A reinvestigation of the mechanical history on superplasticity of Zn-22Al-2Cu at room temperature” by G. Toress-Villasenor, et al. (Material Science Forum, Vol. No. 243/245, p. 553, 1997)

[Non-patent Document 3]

“Fabrication of submicrometer-grained Zn-22% Al by torsion straining” by M. Furukawa, et al., J. Mater. Res., Vol. No.11, No. 9, p. 2128 (1996)

SUMMARY OF THE INVENTION

Those Zn—Al alloys described still have problems to be resolved. One of the problems lies in the fact that the Zn—Al alloy exhibiting excellent superplasticity at room temperature undergoes deterioration in its properties, particularly in elongation, at a low temperature, thereby causing deterioration in superplasticity. Accordingly, a new problem has arisen in that the Zn—Al alloy is unsuitable for the constructional damper, and so forth, for use at low temperatures in cold climates, or the like.

Further, besides the problem described as above, there has newly arisen another problem that as the constructional damper, and so forth increase in size, so inevitably does the size of an ingot of the Zn—Al alloy, and in the case of a larger-sized ingot of the Zn—Al alloy, there results unevenness in properties, such as hardness, and so forth, particularly between the central part and surface layer zone thereof.

The invention has been developed to address those problems, and it is therefore an object of the invention to provide a Zn—Al alloy excellent in elongation, and a method for producing the same.

To attain the object, in one aspect, the invention provides a Zn—Al alloy excellent in elongation, comprising Zn in a range of 68 to 88% by mass, and the remainder including Al and unavoidable impurities, the Zn—Al alloy having a structure with β phases finely dispersed in respective α phases or respective α′ phases, not more than 5 μm in average grain size, a macrosegregation value of Al in the structure being less than 3%, wherein lamellar structures at a central part of the structure is at not higher than 30% by volume, and a difference in average hardness between the central part and a surface layer zone of the structure is not more than 15%.

To attain the object, in another aspect, the invention provides a method of producing a Zn—Al alloy excellent in elongation by pouring molten metal of the Zn—Al alloy comprising Zn in a range of 68 to 88% by mass, and the remainder including Al and unavoidable impurities, into an ingot mold, the method comprising a process for casting while shielding poured molten metal from an external atmosphere, an ingot mold-cooling process after the casting for cooling the ingot in a temperature range of 425 to 375° C. at an average cooling rate of not less than 1.0° C./sec, and subsequently cooling the ingot in a temperature range of 275 to 250° C. at an average cooling rate of not less than 0.08° C./sec, a reheating process for reheating the ingot so as to be heated up and held at not lower than 350° C. before being quenched at an average cooling rate of not less than 0.5° C./sec, and a warm working process for executing warm working at a temperature not higher than 275° C., and at an extrusion ratio of not less than 4.

For the Zn—Al alloy to exhibit superplasticity, the Zn—Al alloy needs to have a structure in which β phases are dispersedly precipitated in a phases or α′ phases (hereinafter referred to also as α-phases with β-phases dispersed therein). The a phases with the β phases dispersed therein completely differ from the α phases without the β phases precipitated therein and can exhibit elongation of 200% or higher owing to plastic deformation due to shift of crystal grains.

Accordingly, an ingot of the Zn—Al alloy is normally reheated to be subsequently quenched so as to leave out the β phases as precipitated in the respective α phases, thereby obtaining the structure of the α phases with the β phases dispersed therein, capable of exhibiting the superplasticity. If a cooling rate is low upon quenching, lamellar structures with the β phases diffused therein are formed, and in the case of a low processing ratio in a working process, the α phase and the β phase are susceptible to insufficient micronization, so that elongation at room temperature remains low in a range of about 100 to 140%.

The description as above is disclosed in Patent Document 3 as previously referred to, however, Patent Document 3 has no recognition on relations between formation of the lamellar structures at the central part of a structure and deterioration in elongation, although it has recognition on relations between formation of the lamellar structures and deterioration in elongation at room temperature.

With the invention, formation of the lamellar structures, particularly in the central part of the structure causing deterioration in elongation at low temperature, is controlled, thereby obtaining the Zn—Al alloy excellent in elongation at low temperature as well as at room temperature. Further, the formation of the lamellar structures in the central part of the structure will lead to elimination of a problem, that there occurs unevenness in characteristics between the central part and the surface part of a large-sized ingot of the Zn—Al alloy. Thus, the Zn—Al alloy excellent in uniformity as well can be obtained.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing a structure of a Zn—Al alloy according to a comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Structure as Precondition)

First, there is described hereinafter a precondition for a structure of a Zn—Al alloy according to the invention. With the present invention, it is assumed as a precondition for the Zn—Al alloy to exhibit superplasticity that the structure has β phases dispersedly precipitated in α phases or α′ phases (the α-phases with β-phases dispersed therein) as previously described, also as described with reference to Patent Documents 1 to 3. The α phases without the β phases precipitated therein do not exhibit superplasticity, however, the α phases with the β phases dispersed therein can exhibit elongation of 200% or higher at room temperature, owing to plastic deformation due to shift of crystal grains.

Herein, the α phase is defined as a crystal of a face-centered cubic lattice, with Al as the main constituent thereof, the α′ phase as a crystal of a face-centered cubic lattice, with Zn as the main constituent thereof, and the β phase as a crystal of a hexagonal close-packed lattice, with Zn as the main constituent thereof.

A metal structure largely varies depending on Zn content. A metal structure exhibiting superplasticity is obtained by a Zn—Al alloy containing Zn in a range of 68 to 88 mass % on the precondition that a preferable method for fabrication, to be described later, is adopted. Macroscopically, this Zn—Al alloy has an a single-phase structure with the β phases finely dispersed in the respective α phases or the respective α′ phases.

In contrast, if Zn content is in excess of 88 mass %, the β phases are not finely dispersed in the respective α phases or the respective α′ phases. In such a case, a structure will macroscopically become a mixed structure of an (α+β) dual phase, exhibiting no superplasticity. Further, “macro α phase” and “macro β phase” each refer to a structure that can be recognized by observation with a microscope (magnification: on the order of 1000 times). In contrast, the β-phases microscopically precipitated in the α-phases with the β-phases dispersed therein, according to the invention, refers to structures that can be recognized by observation with a microscope of a magnification over approximately 5000 times. Accordingly, the β-phases microscopically precipitated in the α-phases according to the invention are definitely distinguishable from the macro β phase described as above.

In the case of a Zn—Al alloy with Zn content in excess of 88 mass %, the macro β phase in the mixed structure of the (α+β) two phases simply exhibits ductility on the order of 65% in a recovery phenomenon at room temperature. On the other hand, in the case of the Zn—Al alloy containing 68 to 88 mass % Zn, the α-phases with the β-phases dispersed therein exhibit elongation of over 200%, and occurrence of concentration of stress at a grain boundary face of each of the β-phases can be avoided, thereby exhibiting elongation of over 160% as a whole.

In contrast, with the dual phase structure (α+β) consisting of the α-phase without precipitation of the β phase therein, and the β phase, such as the structure for the Zn—Al alloy with Zn content in excess of 88 mass %, ductility for the α-phase, and ductility for the β phase, respectively, are simply exhibited, so that it is not possible to exhibit superplasticity. Further, the recovery phenomenon at room temperature occurs to the macro β phase, thereby stabilizing deformation resistance; however, the macro β phase has elongation on the order of 65%. Thus, with the dual phase structure (α+β) of the α-phase with no precipitation of the β phase and the β phase, elongation as a whole remains around 68%.

In this connection, the α-phases without precipitation of the β phases, and the macro β phases are preferably not present in the structure, however, provided that the structure has the α-phases with the β-phases dispersed therein, capable of exhibiting superplasticity, the macro β phases may be present as far as presence thereof does not interfere with superplasticity.

(Average Grain Size)

With the α phase or the α′ phase, each as a main phase, or the β phase precipitated in the α phase or the α′ phase, the average grain size thereof is preferably not more than 5 μm, more preferably not more than 3.5 μm. The finer the respective average grain sizes of the α phase or the α′ phase and the β phase precipitated therein are, the easier the exhibition of superplasticity becomes. On the other hand, if the respective average grain sizes exceed 5 μm, it becomes difficult to exhibit the superplasticity at room temperature (static deformability) with elongation of over 160% at room temperature.

(Lamellar Structure)

With the present invention, lamellar structures at the central part of the structure are controlled so as to be kept at not higher than 30% by volume in order to enhance elongation at low temperature and room temperature. When an ingot of the Zn—Al alloy is re-heated to be subsequently quenched, a cooling rate at the central part inevitably becomes lower in comparison with that in a surface layer zone. For this reason, the lamellar structures with the β phases dispersed therein are prone to occur to the central part. This is the reason why the lamellar structures are prone to be formed at the central part, causing deterioration in elongation including elongation at low temperature.

Further, the formation of the lamellar structures, at the central part, causes unevenness in structure and property to occur between the central part and the surface layer zone. Accordingly, there arises a problem of unevenness in property, particularly, in hardness between the central part and the surface of the ingot. Therefore, controlling the lamellar structures in the central part to not be higher than 30% by volume contributes to enhancement in thicknesswise uniformity in hardness and so forth of a plate. Consequently, it becomes possible to even out the central part and the surface layer zone in respect of hardness such that a difference in average hardness therebetween is not more than 15%. Meanwhile, if the lamellar structures in the central part are controlled to not be higher than 30% by volume, the lamellar structures on the part of the surface layer zone can inevitably be controlled to not be higher than 30% by volume.

In this case, respective lamellar intervals of the lamellar structure were found to be not more than 1000 nm. A lamellar interval dimension was obtained by measuring a portion of the visual field, about 35 μm×about 25 μm, on the basis of the reflection electron images magnified 3000 times by the SEM, where the lamellar interval is at the maximum. The lamellar interval refers to spacing of each of respective boundaries between white parts and black parts of the respective lamellar structures shown in the photograph.

Now, with the present invention, the central part refers to a part of the Zn—Al alloy within a distance from a center position of the Zn—Al alloy, equivalent to not more than 10% of a distance between the center position and the outermost part of the Zn—Al alloy. The center position refers to the center in a cross section of the alloy in the radial direction, if the alloy is, for example, a round bar stock. If the alloy is a square bar stock, the center position refers to a point of intersection of diagonal lines of a cross section, and if the alloy is in other shapes, the center position generally refers to a position in a crosssection, at the center of gravity. Further, if the alloy is a plate stock, the center position refers to a thicknesswise center of the plate stock. Similarly, with the present invention, the surface layer zone refers to a part of the Zn—Al alloy within a distance from the surface layer, equivalent to not more than 10% of the distance between the center position and the outermost part of the Zn—Al alloy.

As described in the foregoing, the lamellar structure can be observed as a structure having a layered structure several to several tens of microns in size, present in the α phases with the β phases dispersed therein, by SEM (Scanning Electron Microscope) of a magnification over 3000 times. Therefore, the lamellar structure can be definitely distinguished from the α phase or the α′ phase that is granular in form. In FIG. 1, there is shown a structure of Comparative Example 8 (lamellar structures at the central part represents 25% by volume) in Table 2 for the embodiments, described later, as observed by a 3000× SEM. In FIG. 1, structures each having a layered structure, indicated by symbol O, are the lamellar structures. In contrast, white granular structures around the respective lamellar structures are the β phases of zinc, and black granular structures around the respective lamellar structures are the α phases or the α′ phases of aluminum.

To obtain a percentage by volume of the lamellar structures in the central part, observation is made on respective microstructures in ten visual fields, each about 35 μm×about 25 μm in size, in the central part of the Zn—Al alloy, on the basis of reflection electron images magnified 3000 times by the SEM. Subsequently, after tracing or image processing of the lamellar structures that can be definitely distinguished as described above, percentages by volume of the lamellar structures in the respective visual fields are measured to be thereby averaged out in the ten visual fields.

(Macrosegregation of Al)

Macrosegregation of Al refers to segregation occurring to the top and bottom of an ingot of the Zn—Al alloy, and thicknesswise thereof. With the present invention, the macrosegregation of Al is to be less than 3.0% in order to render dynamic deformability of the Zn—Al alloy excellent. With reference to the macrosegregation of Al, an ingot cast from a Zn—Al alloy warm working stock as obtained is cut at its top and bottom, respectively, and a test specimen is taken from four spots at the central part and surface layer zone, in respective cut faces (section) to thereby measure Al contents (concentration) at respective spots. The Al contents whose discrepancy from the Al content (average) of the Zn—Al alloy is at a maximum value is regarded as representing the macrosegregation, and a difference from the Al content (average) of the Zn—Al alloy (the Al content at a maximum value minus the Al content) is found as a macrosegregation value of Al. If the macrosegregation value of Al is found at 3.0% or higher, there occurs deterioration in dynamic deformability at room temperature and low temperature.

(Al Inclusions)

In order to render dynamic deformability of the Zn—Al alloy excellent, it is preferable to decrease coarse inclusions in the Zn—Al alloy. The coarse inclusions are mainly Al-based inclusions such as Al₂O₃, and so forth, however, all the coarse inclusions without being limited to the Al-based inclusions need be controlled. Those coarse inclusions each act as a starting point of rupture, thereby causing deterioration in not only dynamic deformability but also static deformability. Hence, in order to prevent deterioration in such properties as described, it is preferable to control a maximum diameter of each of the inclusions in a structure to not more than 50 μm in terms of the equivalent of a circle diameter.

(Pores)

In order to render dynamic deformability of the Zn—Al alloy excellent, it is preferable that pores (vacancy) not less than 0.5 mm in diameter in terms of the equivalent of a circle diameter are not present. The pores each also act as a starting point of rupture, thereby causing deterioration in not only dynamic deformability but also static deformability. Hence, in order to prevent deterioration in such properties as described, it is preferable to control the maximum diameter of each of the pores in a structure to less than 0.5 mm in terms of the equivalent of a circle diameter.

(Chemical Composition)

Chemical composition of the Zn—Al alloy according to the invention is described hereinafter. According to the chemical composition of the Zn—Al alloy according to the invention, Zn content is in a range of 68 to 88% by mass, and the remainder includes Al and unavoidable impurities. As previously described, the metal microstructure of an alloy largely varies depending on Zn content. On the precondition of the preferable method for fabrication, to be described later, the metal microstructure exhibiting the superplasticity is obtained by the Zn—Al alloy of the chemical composition described as above. Macroscopically, the Zn—Al alloy has the structure of the α single phase, with the β phases finely dispersed in the respective α phases or the respective α′ phases. The α-phases with the β-phases dispersed therein exhibits elongation of over 200%, and the occurrence of concentration of stress, at the grain boundary face of the β-phase, can be avoided, thereby exhibiting elongation of over 160% as awhole.

In contrast, if Zn content is in excess of 88 mass %, the β phases are not finely dispersed in the respective α phases or the respective α′ phases as previously described. In such a case, a structure will macroscopically become the mixed structure of the (α+β) two phases. The recovery phenomenon at room temperature occurs to the macro a phase, thereby stabilizing the deformation resistance; however, the macro a phase simply exhibits ductility on the order of 65% in the recovery phenomenon at room temperature. Thus, the mixed structure of the (α+β) two phases of the α phase and the β phase simply exhibits the respective ductilities of the a phase and the β phase, and will be a structure exhibiting elongation as a whole at around 68%, and exhibiting no superplasticity.

Meanwhile, as Zn content decreases within the range described as above, so does precipitation of the β-phase, so that elongation tends to decrease even if there occurs plastic deformation due to shift of crystal grains. In the case of Zn content being less than 68 mass %, elongation in excess of 100% cannot be exhibited even if processed on the condition according to the invention.

If requirements described as above are fulfilled, the Zn—Al alloy according to the invention may contain Cu, Si, Mn, and Mg as reinforcement elements so as to prevent standing stress from undergoing excessive change owing to magnitude of working as applied and a strain rate to the extent that hysteresial stability is not impaired. Further, in order to enhance elongation, the Zn—Al alloy according to the invention may contain Zr, Ti, and B, effective for crystal micronization.

(Production Method)

There is described hereinafter a preferable method of production for efficiently obtaining the Zn—Al alloy according to the invention, fulfilling the requirements described, and excellent in elongation.

The preferable method of production is by pouring molten metal of the Zn—Al alloy of the composition described as above into an ingot mold, the method of the production comprising main processes including a process of casting while shielding poured molten metal from an external atmosphere, an ingot mold-cooling process after the casting, a reheating process, and a warm working process.

(Process of Casting)

At the time of production by pouring the molten metal of the Zn—Al alloy into the ingot mold, the poured molten metal is preferably shielded from the external atmosphere. As a result of the casting while shielding the poured molten metal from the external atmosphere, bond with oxygen can be controlled, so that it becomes possible to check growth of coarser grains of oxide inclusions such as Al₂O₃, and so forth. Consequently, the inclusions based on Al, and so forth can be controlled to be not more than 50 μm in maximum diameter in terms of the equivalent of a circle diameter. As a specific method of shielding, it is effective to adopt a vacuum atmosphere, or an Ar gas atmosphere (Ar sealing) as an ambient atmosphere at the time of casting, or to immerse an injection nozzle in molten metal (nozzle immersion).

(Ingot Mold-Cooling Process Step 1 After the Casting)

In an ingot mold-cooling step after the casting, an ingot in a temperature range of 425 to 375° C. is first cooled at an average cooling rate of not less than 1.0° C./sec. By cooling the ingot in the temperature range of 425 to 375° C., corresponding to a solid-liquid dual phase region, at the average cooling rate of not less than 1.0° C./sec during the ingot mold-cooling step after casting, it is possible to check formation of a coarse solidification structure causing macrosegregation. That is, by cooling the ingot in the temperature range described as above, at a relatively high cooling rate, Al crystal grains grow coarser, thereby checking the formation of the coarse solidification structure.

(Ingot Mold Cooling Process Step 2 After the Casting)

In the ingot mold-cooling step after the casting, the ingot in a temperature range of 275 to 250° C. is subsequently cooled at an average cooling rate of not less than 0.08° C./sec. By cooling the ingot in the temperature range of 275 to 250° C., corresponding to the (α+β) dual phase, at the average cooling rate of not less than 0.08° C./sec during the ingot mold-cooling step after casting, it is possible to check precipitation of coarse β phases, thereby checking microsegregation caused mainly by the coarse β phase present in the α phases. That is, by cooling the ingot in the temperature range described at a relatively high cooling rate, it is possible to check coarser precipitation of Zn and Al to check formation of the coarse β phases, thereby obtaining the β phases finely dispersed.

(Reheating Process)

In the reheating process, the ingot cooled down to room temperature or not higher than 250° C. in the ingot mold-cooling step is reheated so as to be heated up and held at not lower than 350° C. before being quenched. By increasing the cooling rate in the ingot mold-cooling step, as described above, it is possible to check, to a degree, the formation of the coarse solidification structure, however, for further control of the formation of the coarse solidification structure, it is effective to apply reheating after the ingot mold-cooling step to thereby homogenize the ingot.

In order to attain sufficient homogenization, a soaking temperature is preferably set to 350° C. or higher. However, the temperature is preferably set to less than 390° C. because of the risk of the ingot being melted if the temperature is at 390° C. or higher.

Further, holding time at the heating temperature described in the reheating process, on the order of one hour, is sufficient for homogenization in the case of a small-sized ingot weighing, for example, 50 kg or less, however, in the case of a large-sized ingot weighing, for example, 150 kg or more, it takes as long as 8 hours or more to cause the alloy in whole to reach 350° C. or higher. This is because the ingot absorbs heat from outside due to an excessively large endotherm at the time of β particles undergoing solid solution into an a matrix again, so that in the case of the large-sized ingot, there is no choice but to heat up an atmosphere for many hours. Accordingly, adoption of radio-frequency heating is conceivable. With the radio-frequency heating, since forced heating is executed, heating for many hours is not required, but in the case of the large-sized ingot, the adoption of the radio-frequency heating will cause an increase in cost from a commercial point of view.

After heating up and holding the ingot at not lower than 350° C., the ingot is quenched at an average cooling rate of not less than 0.5° C./sec. By holding the ingot at not lower than 350° C., at the time of the reheating, the β phase is trapped in the α phases to thereby prevent microsegregation, and in order to obtain the alloy of the α-phases with the β-phases dispersed therein, capable of exhibiting superplasticity after cooling, it is necessary to quench the ingot at the average cooling rate of not less than 0.5° C./sec after the reheating (soaking). Quenching is carried out down to room temperature, but may be carried out down to a warm working temperature as described later instead of down to room temperature.

By the quenching, it is possible to control conversion of the α′ phase to the α phase that is stable to thereby control diffusion of the β phases to the extent of two-phase separation at a macroscopic level. As a result, the β phases can be left as precipitated in the respective α phases to thereby obtain a structure of the α phases with the β phases dispersed therein, capable of exhibiting the superplasticity. In order to implement the quenching at the average cooling rate of not less than 0.5° C./sec, water cooling is preferable. It is difficult to obtain the average cooling rate of not less than 0.5° C./sec by furnace cooling or air cooling, in which case the β phases undergo diffusion, thereby forming the lamellar structures. If the lamellar structures are formed in this stage, the α phase and the β phase are susceptible to insufficient micronization in the case of a low processing ratio in the following working process, and elongation at room temperature remains in a range of about 100 to 140%, so that elongation of over 160% cannot be reliably achieved.

(Blooming Process)

After the reheating (soaking) and the quenching, blooming is executed selectively at a temperature of 275° C. or lower. With the production method according to the invention, however, it is recommendable to dispense with blooming to thereby execute a warm working process from the viewpoint of process efficiency. Even without execution of the blooming, the structure control that has been attained by conventional blooming can be attained by the warm working process.

In the case of executing the blooming, if a process temperature exceeds 275° C., this will involve a risk in that a structure undergoes transformation, and the α phases with the β phases dispersed therein as once formed will revert to the dual phase structure consisting of the α phase or the α′ phase, and the β phase. For this reason, the blooming is executed preferably at a temperature 200° C. or lower. On the other hand, if the process temperature is too low, this will pose a possibility that processing cracks develop, so that it is recommendable to execute the blooming at a temperature not lower than 100° C. Further, cooling after the blooming is preferably applied at a cooling rate of not less than about 3° C./sec. The reason for this is to lock the α phases with the β phases dispersed therein that have been obtained as with the case of the quenching after the reheating, and more specifically, water cooling is preferably applied.

(Warm Working Process)

The warm working is executed either after the reheating (soaking) and the quenching without execution of the blooming, or after the blooming. The means for the warm working process include forging, extruding, wire-drawing, and so forth, and the extruding is preferably adopted from the viewpoint of production efficiency.

As conditions for the extruding, the warm working is preferably executed on condition of a temperature not higher than 275° C., and at an extrusion ratio of not less than 4. If a warm working temperature exceeds 275° C., this will involve a risk in that a structure undergoes transformation, and the a phases with the β phases dispersed therein as once formed will revert to the dual phase structure consisting of the α phase or the α′ phase, and the β phase. For this reason, the warm working is executed preferably at a temperature 200° C. or lower. On the other hand, if the warm working temperature is too low, this will pose a possibility that processing cracks develop, so that it is recommendable to execute the warm working at a temperature not lower than 100° C.

The extrusion ratio at the warm working is set to not less than 4 in order to cause elongation that can be regarded as the superplasticity at room temperature to be exhibited by applying an external physical force. More specifically, in the case where the blooming is not executed, in a stage where the ingot is subject to the quenching after the soaking, the α phases with the β phases dispersed therein have already been obtained, and the α phase or the α′ phase is in a range of about 10 to 2 μm while the β phase in the α phase or the α′ phase is in a range of about 0.05 to 0.1 μm. Such a structure as described exhibits elongation of over 180%, regarded as the superplasticity, in a high temperature range of about 100 to 150° C., but does not exhibit such elongation as described above at room temperature.

Accordingly, in order to cause the elongation regarded as the superplasticity at room temperature to be exhibited, it is necessary to apply an external physical force (strain produced) to the ingot after the soaking and the quenching to thereby micronize crystal grains of the α phase or the α′ phase, together with crystal grains of the β phase present in the α phase or the α′ phase while crushing the pores. This is the reason why the warm working is executed at the extrusion ratio of not less than 4 after the soaking and the quenching. If the extrusion ratio at the warm working is less than 4, strain produced is too small, so that such a structure described exhibits the elongation regarded as the superplasticity, in the high temperature range, but does not exhibit the elongation regarded as the superplasticity at room temperature as with the above-described structure subjected to the quenching after the soaking.

It is sufficient to apply cooling after the warm working at a cooling rate of not less than about 3° C./sec down to room temperature. More specifically, water cooling is preferably applied. The reason for this is to lock the α phases with the β phases dispersed therein that have been obtained as with the case of the quenching after the reheating, and if the cooling rate is less than about 3° C./sec, there occurs coarsening of the α phases with the β phases dispersed therein, so that the structure will no longer exhibit the superplasticity at room temperature.

With the warm working according to the present invention, supermicronization of the structure can be achieved, so that cold working may be executed, however, there is no particular need for that.

(Joining)

Since the Zn—Al alloy according to the invention has hardness substantially equivalent to, or slightly lower than that of mild steel, the same can be used for application in common joining techniques such as bolting, riveting, and so forth, and joining of the same with building constructions can be easily implemented. However, in the case of executing joining by applying heat such as in the case of soldering, there is the need for controlling a heating temperature to not higher than 250° C., preferably not be higher than 100° C. This is because there is the risk of the structure undergoing transformation at 250° C. or higher, and there are cases where there occurs coarsening of the microstructure purposely obtained unless quenched after heating to 100° C. or higher, thereby rendering it difficult to secure elongation of over 160%.

Now, embodiments of the invention are described in more detail hereinafter, however, it is to be pointed out that the invention is not limited to the embodiments described hereinafter, and that changes and modifications in design, in light of the teachings described hereinbefore and hereinafter, be construed broadly within the spirit and scope of the invention.

Embodiments

(Production of Zn—Al Alloys)

Continuous casting of molten metal of Zn—Al alloys (each has total content of impurities, such as Fe, Cu, Si, Mn, Mg, Zr, Ti, and B, not more than 0.5 mass %) of respective chemical compositions shown in Table 1 was executed with a diameter 142 to be cooled at respective cooling rates shown in Table 1 and Zn—Al alloy ingots (ingot size: 180 kg) were obtained.

The cooling rates of the ingots were found by measuring time-dependent change (a cooling curve) in internal temperatures of the respective ingots by use of a thermocouple installed at a position (a center position in section) 300 mm away from the bottoms of the respective ingots. Subsequently, the average cooling rate (average cooling rate 1) in the solid-liquid dual phase region (425 to 375° C.), and the average cooling rate (average cooling rate 2) at a β-phase-precipitation starting temperature (275 to 250° C.) were worked out from the cooling curve.

Further, for sealing of the ingot (sealing of the ingot from the external atmosphere), the Ar sealing is applied to the interior and tuyere of the ingot molds prior to ingot charging. In the case of the continuous casting, the sealing was provided by immersing the injection nozzle in molten metal (nozzle immersion).

The Zn—Al alloy ingots as obtained were reheated (soaked) in an atmospheric air furnace to respective temperatures shown in Table 1, were held for eight hours at those temperatures. Such holding time as described refers to a length of time elapsed from a time when respective ingot temperatures reach predetermined heating temperatures after the thermocouple is brought into contact with the surfaces of the respective ingots in the atmospheric air furnace.

After the reheating, and immediately after the respective ingots were taken out of the air furnace, the respective ingots were cooled down to temperatures for isothermal extrusion, which is the warm working, at respective average cooling rate shown in Table 1. Thereafter, the isothermal extrusion at respective warm working temperatures shown in Table 1 was applied to the respective ingots to be subsequently cooled at respective average cooling rate shown in Table 1, whereupon bar stocks of Φ60 and Φ36, respectively, were obtained.

The Zn—Al alloy bar stocks thus obtained were evaluated in respect of structure, room temperature characteristics, and low temperature characteristics, respectively, by the following method. Results of evaluation are shown in Table 2. Herein, the central part refers to a part of the Zn—Al alloy bar stock within a distance from a center position of the bar stock, equivalent to not more than 10% of a distance between the center position and the outermost part of the Zn—Al alloy bar stock, as previously described. The center position refers to the center in a cross section of the bar stock, in the radial direction. Further, the surface layer zone of the bar stock refers to a part of the Zn—Al alloy bar stock within a distance from the surface layer of the bar stock, equivalent to not more than 10% of the distance between the center position and the outermost part of the bar stock, as previously described.

(Structure Determination, α Average Grain Size)

With reference to the Zn—Al alloy bar stocks as obtained, metal structures thereof were observed with an electron microscope to determine whether or not the respective metal structures are the α phases with the β phases dispersed therein, capable of exhibiting the superplasticity, and also to measure respective α (including α′) average grain sizes. Further, specimens were taken from the center part, and observation on the specimens of the alloy, subjected to buffing, was conducted with a SEM (Scanning Electron Microscope) of a magnification of 5000 times, whereupon three microphotographs were taken to thereby determine whether or not the respective metal structures are the α phases with the β phases dispersed therein. Further, the respective α phases were checked in diameter in terms of the equivalent of a circle diameter, and on the assumption that such diameters as found are grain sizes, a mean value of the respective grain sizes in three visual fields was found.

(Lamellar Structure Percentage by Volume)

To obtain a percentage by volume of lamellar structures in the central part, observation was made on respective microstructures in ten visual fields, each about 35 μm×about 25 μm in size, in the central part of the Zn—Al alloy, on the basis of reflection electron images magnified 3000 times by the SEM. Subsequently, the lamellar structures as distinguished were traced. For image processing software, use was made of Image-ProPlus produced by MEDIACYBERNETICS Company, and the percentages by volume of the lamellar structures in the respective visual fields were measured to be thereby averaged out in the ten visual fields.

In this case, respective lamellar intervals of the lamellar structure were found to be not more than 1000 nm. A lamellar interval dimension was obtained by measuring a portion of the visual field, about 35 μm×about 25 μm, on the basis of the reflection electron images magnified 3000 times by the SEM, where the lamellar interval is at the maximum. The lamellar interval refers to spacing of each of respective boundaries between white parts and black parts of the respective lamellar structures shown in the photograph.

(Inclusions)

With respective examples, a maximum diameter of each of inclusions such as the Al-based inclusions was found not more than 7 μm. To obtain the maximum diameters of the respective inclusions, observation by use an optical microscope of a magnification 1000 times was made on a portion of the Zn—Al alloy bar stocks as obtained, with the surface already buffed, at a position 100 mm away from the top thereof, in the direction of extrusion and three microphotographs were taken to thereby determine that a grain size of the inclusion having the maximum grain size (diameter in terms of the equivalent of a circle diameter) among the inclusions as observed (distinguished) is the maximum grain size of all the inclusions.

(Macrosegregation)

An ingot cast from an extruded material of the Zn—Al alloy obtained was cut at its top and bottom, respectively, and a test specimen was taken from four spots at the central part and surface layer zone, in respective cut faces (sections) to thereby measure Al contents (concentration) at the respective spots. The Al contents whose discrepancy from the Al content (average) of the Zn—Al alloy was at a maximum value was regarded as representing the macrosegregation, and a difference from the Al content (average) of the Zn—Al alloy (the Al content at a maximum value minus the Al content) was found as the macrosegregation value of Al.

(Pores)

With any of the examples, pores not less than 0.5 mm in diameter in terms of the equivalent of a circle diameter were not present. In measurement of the pores, samples (50 mm square bars) were taken, and reheated for ten hours to be subsequently subjected to HIP processing, thereby having completely removed the pores. Thereafter, standard test specimens each with a drill hole 0.5 mm in diameter, bored at the center thereof, were prepared to be subjected to UT inspection (ultrasonic flaw inspection), having thereby examined a UT noise level capable of detecting the hole 0.5 mm in diameter. Then, it was determined that the pores not less than 0.5 mm in diameter were present in the test specimen in which there occurred noise at a level higher than the noise level described

(Tensile Test)

The Zn—Al alloy bar stocks obtained were evaluated in respect of the room temperature characteristics, and the low temperature characteristics, respectively. A round bar test specimen 14 mm in diameter of a parallel part thereof, according to JIS No. 4, was taken from the respective Zn—Al alloy bar stocks, and the test specimens were subjected to a tensile test at a gauge length 50 mm, and crosshead speed of 50 mm/min (at a strain rate of 1.67×10⁻² S⁻¹: dynamic deformability) to thereby measure tensile strength TS, elongation upon occurrence of breaking (elongation at break), and contraction of area, whereupon evaluation was made on dynamic characteristics (tensile strength TS, and elongation at break, upon quick deformation) of the respective alloys. The tensile tests were conducted at 25° C. as the room temperature, and −10° C. as the low temperature. The contraction of area was found from a sectional area of a broken part of each of the test specimens after being broken.

As is evident from Table 1 and Table 2, working examples of the invention, 1 to 6, each have chemical composition within a range according to the invention, and were produced within a range of the preferable production conditions. Hence, those working examples each have a structure in which the β phase is finely dispersed in the respective α phases or the respective α′ phases, not more than 5 μm in average grain size, and the macrosegregation value of Al, that is, a difference between the maximum Al content of the alloy and the Al content (average) of the alloy is less than 3.0%, the lamellar structures at the central part of the structure being at not higher than 30% by volume. Accordingly, the working examples are excellent in elongation (superplasticity, and deformability) not only at room temperature but also at low temperature, and are also excellent in uniformity in hardness between the central part and the surface layer zone such that a difference in average hardness therebetween is not more than 15%.

In contrast, comparative examples 7 to 11 are outside the range according to the invention, in respect of either the chemical composition, or the preferable production conditions. Consequently, those comparative examples cannot meet stipulations of the invention in respect of the α phases with the β phases dispersed therein, the average grain size of the α phase, the macrosegregation of Al, and the percentage by volume of the lamellar structures in the central part of a structure. Consequently, the comparative examples are found considerably inferior to the working examples in respect of any of elongation at room temperature, elongation at low temperature, and a difference in average hardness between the central part and the surface layer zone.

The comparative example 7 had an excessively high Zn content. For this reason, the comparative example 7 is found considerably inferior to the working examples in respect of elongation at room temperature, and elongation at low temperature.

With the comparative example 8, both an average cooling rate in the temperature range of 425 to 375° C. during an ingot mold-cooling step after casting, and an average cooling rate in the temperature range of 275 to 250° C. during another ingot mold-cooling step after casting were too low. For this reason, with the comparative example 8, there occurred an increase in percentage by volume of the lamellar structures in the central part of a structure and the comparative example 8 is found considerably inferior to the working examples in respect of elongation at room temperature, and elongation at low temperature.

With the comparative example 9, temperature in a reheating process was too low. For this reason, with the comparative example 9, there occurred an increase in percentage by volume of the lamellar structures in the central part of a structure and the comparative example 9 is found considerably inferior to the working examples in respect of elongation at room temperature and low temperature, and a difference in average hardness between the central part and the surface layer zone.

With the comparative example 10, an average cooling rate after reheating was too low. For this reason, with the comparative example 10, there occurred an increase in percentage by volume of the lamellar structures in the central part of a structure and the comparative example 10 is found considerably inferior to the working examples in respect of elongation at room temperature, and low temperature, and a difference in average hardness between the central part and the surface layer zone.

With the comparative example 11, an extrusion ratio (strain produced) at the time of the warm working was too small. For this reason, the comparative example 11 is found considerably inferior to the working examples in respect of elongation at room temperature, and low temperature.

It can be the that test results described in the foregoing are in support of significance of the chemical composition, and the structure, as stipulated in the invention, and the preferable production conditions for obtaining the structure, on elongation not only at room temperature but also at low temperature, and uniformity. TABLE 1 Average Cooling Rate after Reheating Warm Working (Extruding) Chemical Composition Casting Average Average of Zn—Al alloy Temperature Temperature Temper- Cooling Cooling (mass %) Range of 425 Range of 275 ature Rate Temperature Extrusion Rate Classification No. Zn Impurities Remainder to 375° C. to 250° C. (° C.) (° C./s) (° C.) Ratio (° C./s) Working 1 78 not more than 0.5 Al 2 0.16 350 6 200 7 0.3 Example 2 78 not more than 0.5 Al 2 0.16 350 4 200 7 0.3 3 85 not more than 0.5 Al 2 0.16 350 4 200 7 0.3 4 78 not more than 0.5 Al 1.2 0.9 350 4 200 7 0.3 5 78 not more than 0.5 Al 2 0.16 350 2 200 7 0.3 6 78 not more than 0.5 Al 2 0.16 350 4 200 18 0.3 Comparative 7 90 not more than 0.5 Al 2 0.16 350 4 200 7 0.3 Example 8 78 not more than 0.5 Al 0.6 0.06 350 4 200 7 0.3 9 78 not more than 0.5 Al 2 0.16 320 4 200 7 0.3 10 78 not more than 0.5 Al 2 0.16 350 0.3 200 7 0.3 11 78 not more than 0.5 Al 2 0.16 350 4 200 2 0.3

TABLE 2 Zn—Al alloy bar stock properties Zn—Al alloy bar stock structure at room temperature (25° C.) α phase Percentage by Central Difference Macro- average Volume of Lamellar Surface Part in segregation grain size Lamellar Interval Hardness Hardness Hardness Classification No. Structure (%) (μm) Structures (%) (nm) (Hv) (Hv) (%) Working 1 α-phases with 1.6 1.5 5 Not more 39 41 4.9 Example β-phases than 200 nm dispersed therein 2 α-phases with 1.6 2.5 8 Not more 40 43 7.0 β-phases than 200 nm dispersed therein 3 α-phases with 1.8 2.7 7 Not more 42 46 8.7 β-phases than 200 nm dispersed therein 4 α-phases with 2.5 3.8 10 600 44 48 8.3 β-phases dispersed therein 5 α-phases with 1.6 4.2 14 800 45 52 13.5 β-phases dispersed therein 6 α-phases with 1.6 2.2 8 Not more 38 40 5.0 β-phases than 200 nm dispersed therein Comparative 7 α-phases with 2.0 5.3 8 Not more 42 45 6.7 Example β-phases than 200 nm dispersed therein 8 α-phases with 4.8 2.9 25 Not more 46 52 11.5 β-phases than 200 nm dispersed therein 9 α-phases with 1.7 3.1 35 700 48 57 15.8 β-phases dispersed therein 10 α-phases with 1.7 3.5 40 1500  49 60 18.3 β-phases dispersed therein 11 α-phases with 1.6 7.2 10 1800  44 50 12.0 β-phases dispersed therein Zn—Al alloy bar stock properties Zn—Al alloy bar stock properties at room temperature (25° C.) at low temperature (−10° C.) Tensile Contraction Strain Tensile Contraction Strain Strength Elongation of Rate Strength Elongation of Classification No. Rate (/s) (MPa) (%) Area (%) (/s) (MPa) (%) Area (%) Working 1 0.017 310 72 99.6 0.017 345 65 91 Example 2 0.017 371 48 89 0.017 412 44 81 3 0.017 381 55 92 0.017 423 50 84 4 0.017 390 50 90 0.017 433 45 82 5 0.017 394 47 87 0.017 437 43 81 6 0.017 288 88 99.8 0.017 320 72 92 Comparative 7 0.017 375 42 90 0.017 416 34 81 Example 8 0.017 420 39 75 0.017 466 33 66 9 0.017 430 47 70 0.017 477 41 60 10 0.017 440 46 66 0.017 488 40 54 11 0.017 392 37 80 0.017 435 29 71

As described hereinbefore, the invention can provide a Zn—Al alloy excellent in elongation, and a method of producing the same. As a result, the Zn—Al alloy according to the invention is available for various applications including base isolation/vibration control devices making the most of superplasticity at room temperature and low temperature, such as a constructional damper, and so forth, an impact absorption member of an automobile, and so forth, a spring member having damping capacity, precision-forming components, a sealing member, a gasket member, a foil member requiring superplasticity and deformation property, at room temperature, a target member for thin-film formation, and so forth.

The foregoing invention has been described in terms of preferred embodiments. However, those skilled, in the art will recognize that many variations of such embodiments exist. Such variations are intended to be within the scope of the present invention and the appended claims. 

1. A Zn—Al alloy excellent in elongation, comprising: Zn in a range of 68 to 88% by mass; and the remainder including Al and unavoidable impurities, the Zn—Al alloy having a structure with β phases finely dispersed in respective α phases or respective α′ phases, not more than 5 μm in average grain size, a macrosegregation value of Al in the structure being less than 3.0%, wherein lamellar structures at a central part of the structure is not higher than 30% by volume, and a difference in average hardness between the central part and a surface layer zone of the structure is not more than 15%.
 2. The Zn—Al alloy excellent in elongation according to claim 1, wherein an interval between the respective lamellar structures adjacent to each other is not more than 1000 nm.
 3. The Zn—Al alloy excellent in elongation according to claims 1 or 2, wherein a maximum diameter of each of inclusions in the structure is not more than 50 μm in terms of the equivalent of a circle diameter, and pores not less than 0.5 mm in diameter in terms of the equivalent of a circle diameter are not present.
 4. A method of producing a Zn—Al alloy excellent in elongation by pouring molten metal of the Zn—Al alloy comprising Zn in a range of 68 to 88% by mass, and the remainder including Al and unavoidable impurities, into an ingot mold, the method comprising processes for: casting while shielding poured molten metal from an external atmosphere; cooling an ingot mold after the casting in a temperature range of 425 to 375° C. at an average cooling rate of not less than 1.0° C./sec, and subsequently cooling the ingot mold in a temperature range of 275 to 250° C. at an average cooling rate of not less than 0.08° C./sec; a reheating process for reheating the ingot so as to be heated up and held at not lower than 350° C. before being quenched at an average cooling rate of not less than 0.5° C./sec; and a warm working process for executing warm working at a temperature not higher than 275° C., and at an extrusion ratio not less than
 4. 